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Aim: Repolarization response to β-adrenergic (β-AR) stimulation differs between guinea-pig and canine myocytes and, within the latter, between myocardial layers. Correlative analysis suggests that this may be due to differences in action potential (AP) contour. Here we tested whether AP contour may set the response of current and of repolarization to β-AR stimulation (10 nM isoproterenol, ISO).Methods and results: The responses of AP and current to ISO were measured under I-clamp and "AP-clamp" in guinea-pig (GP), dog epicardial (DEPI) and dog subendocardial (DENDO) myocytes. Dynamic-clamp (DC) was used to evaluate the impact of AP features on AP response to ISO. ISO prolonged AP duration (APD) in GP myocytes, did not affect it in DENDO and shortened it in DEPI ones. The current induced by ISO (IISO) sharply differed between GP and canine myocytes and, to a lesser extent, between DENDO and DEPI ones. Differences in IISO profile likely important in setting APD response (time-to-peak, time-to-reversal), were minimized when canine myocytes where clamped with GP AP-waveforms and vice versa. Introduction of a "notch" in GP AP (by DC) was alone insufficient to affect the APD response to ISO; nevertheless, when incorporated in a GP AP-waveform, the main "canine" AP features ("notch" and low plateau potential) caused IISO of GP myocytes to acquire canine features.Conclusion: Early repolarization contour and level of plateau potential contribute to species-specificity of IISO profile. Changes in AP contour, also when generated by modulation of ISO-insensitive currents, may be crucial in setting APD response to β-AR stimulation. [ABSTRACT FROM AUTHOR]

SEA0400 is a recently developed inhibitor of the Na+/Ca2+ exchanger (NCX) shown to suppress both forward and reverse mode operation of NCX. Present experiments were designed to study the effect of partial blockade of NCX on Ca handling and contractility in Langendorff-perfused guinea pig hearts loaded with the fluorescent Ca-sensitive dye fura-2. Left ventricular pressure and intracellular calcium concentration ([Ca2+]i) were synchronously recorded before and after cumulative superfusion with 0.3 and 1 μM SEA0400. SEA0400 caused no significant change in the systolic and diastolic values of left ventricular pressure and [Ca2+]i. Accordingly, pulse pressure and amplitude of the [Ca2+]i transient also remained unchanged in the presence of SEA0400. SEA0400 had no influence either on the time required to reach peak values of pressure and [Ca2+]i or on half relaxation time. On the other hand, both 0.3 and 1 μM SEA0400 significantly increased the decay time constant of [Ca2+]i transients, obtained by fitting its descending limb between 30% and 90% of relaxation, from 127 ± 7 to 165 ± 7 and 177 ± 14 ms, respectively ( P < 0.05, n = 6). In contrast to the guinea pig hearts, rat hearts responded to SEA0400 treatment with increased [Ca2+]i transients and contractility. These interspecies differences observed in the effect of SEA0400 can be explained by the known differences in calcium handling between the two species. [ABSTRACT FROM AUTHOR]

The role of the slow delayed rectifier K+ current ( I Ks) in cardiac repolarization seems to be largely influenced by the experimental conditions including the species and tissue studied. The aim of this study was to determine the contribution of I Ks to repolarization in canine ventricular myocytes by measuring the frequency dependent action potential lengthening effect of 10 μM chromanol 293B using sharp microelectrodes. Pretreatment with isoproterenol (2 nM), E-4031 (1 μM), and injection of inward current pulses were applied to modify action potential configuration. Chromanol alone caused moderate but statistically significant lengthening of action potentials at cycle lengths longer than 500 ms. The lengthening effect of chromanol, which was strongly enhanced in the presence of either isoproterenol or E-4031, was proportional to the amplitude of plateau, whereas poor correlation was found with action potential duration. Similar results were obtained when action potential configuration was modified by injection of depolarizing current pulses. Computer simulations revealed that activation of I Ks is a sharp function of the plateau amplitude within the physiological range, while elongation of repolarization may enhance I Ks only when it is excessive. It was concluded that the effect of I Ks on ventricular repolarization critically depends on the level of action potential plateau; however, other factors, like action potential duration, cycle length, or suppression of other K+ currents can also influence its contribution. [ABSTRACT FROM AUTHOR]

Activators of the slow delayed rectifier K+ current (IKs) are promising tools to suppress ventricular arrhythmias originating from prolongation of action potentials. A recently synthesized compound, L-364,373, was shown to activate IKs in ventricular cells isolated from guinea pigs and rabbits. Due to the interspecies differences known to exist in the properties of the delayed rectifier K+ currents, the effect of L-364,373 on IKs was studied and compared with that of another IKs activator mefenamic acid in canine ventricular myocytes. Mefenamic acid (100 μM) significantly increased the amplitude of the fully activated IKs current, as well as the IKs current tails, by shifting the voltage dependence of its activation towards negative voltages and increased the time constant for deactivation. In contrast, L-364,373, up to concentrations of 3 μM, failed to augment IKs at any membrane potential studied, but slightly increased the time constant of deactivation. It is concluded that human studies are required to evaluate the therapeutically beneficial effects of IKs activators. Rodent cardiac tissues are not suitable for this purpose. [ABSTRACT FROM AUTHOR]

SEA0400 and KB-R7943 are compounds synthesised to block transsarcolemmal Na+/Ca2+ exchange current (INa/Ca); however, they Have also been shown to inhibit L-type Ca2+ current (ICa). The potential value of these compounds depends critically on their relative selectivity for INa/Ca over ICa. In the present work, therefore, the concentration-dependent effects of SEA0400 and KB-R7943 on INa/Ca and ICa were studied and compared in canine ventricular cardiomyocytes using the whole-cell configuration of the patch clamp technique. SEA0400 and KB-R7943 decreased INa/Ca in a concentration-dependent manner, having EC50 values of 111±43 nM and 3.35±0.82 μM, when suppressing inward currents, while the respective EC50 values were estimated at 108±18 nM and 4.74±0.69 μM in the case of outward current block. SEA0400 and KB-R7943 also blocked ICa, having comparable EC50 values (3.6 μM and 3.2 μM, respectively). At higher concentrations (10 μM) both drugs accelerated inactivation of ICa, retarded recovery from inactivation and shifted the voltage dependence of inactivation towards more negative voltages. The voltage dependence of activation was slightly modified by SEA0400, but not by KB-R7943. Based on the relatively good selectivity of submicromolar concentrations of SEA0400—but not KB-R7943—for INa/Ca over ICa, SEA0400 appears to be a suitable tool to study the role of INa/Ca in Ca2+ handling in canine cardiac cells. At concentrations higher than 1 μM, however, ICa is progressively suppressed by the compound. [ABSTRACT FROM AUTHOR]

Objectives: The aim of the present study was to assess and compare the dynamics of L-type Ca2+ current (ICa,L) during physiologic action potential (AP) in canine ventricular cardiomyocytes of epicardial (EPI) and endocardial (ENDO) origin. Methods: ICa,L was recorded on cells derived from the two regions of the heart using both AP voltage clamp and conventional whole cell voltage clamp techniques. Results: AP voltage clamp experiments revealed that the decay of ICa,L is monotonic during endocardial AP, whereas the current is double-peaked (displaying a second rise) during epicardial AP. The amplitude of the first peak was significantly greater in ENDO (−4.6±0.8 pA/pF) than in EPI cells (−2.8±0.3 pA/pF). Application of epicardial APs as command pulses to endocardial cells yielded double-peaked ICa,L profiles, and increased the net charge entry carried by ICa,L during the AP from 0.187±0.059 to 0.262±0.056 pC/pF (n=5, P<0.05). No differences were observed in current densities and inactivation kinetics of ICa,L between EPI and ENDO cells when studied under conventional voltage clamp conditions. Nisoldipine shortened action potentials and eliminated the dome of the epicardial AP. Conclusion: ICa,L was shown to partially inactivate before and deactivate during phase-1 repolarization and reopening of these channels is responsible for the formation of the dome in canine EPI cells. The transmural differences in the profile of ICa,L could be well explained with differences in AP configuration. [Copyright &y& Elsevier]

Background: Thymol is widely used as a general antiseptic and antioxidant compound in the medical practice and industry, and also as a stabilizer to several therapeutic agents, including halothane. Thus intoxication with thymol may occur in case of ingestion or improper anesthesia. In the present study, therefore, concentration-dependent effects of thymol (30-600 micro-grams) were studied on calcium and potassium currents in enzymatically isolated rat skeletal muscle fibers using the double vaseline gap voltage clamp technique. Results: Thymol suppressed both Ca and K currents in a concentration-dependent manner, the EC50 values were 193 ± 26 and 93 ± 11 µM, with Hill coefficients of 2.52 ± 0.29 and 1.51 ± 0.18, respectively. Thymol had a biphasic effect on Ca current kinetics: time to peak current and the time constant for inactivation increased at lower (100-200 µM) but decreased below their control values at higher (600 µM) concentrations. Inactivation of K current was also significantly accelerated by thymol (200-300 µM). These effects of thymol developed rapidly and were partially reversible. In spite of the marked effects on the time-dependent properties, thymol caused no change in the current-voltage relationship of Ca and K peak currents. Conclusions: Present results revealed marked suppression of Ca and K currents in skeletal muscle, similar to results obtained previously in cardiac cells. Furthermore, it is possible that part of the suppressive effects of halothane on Ca and K currents, observed experimentally, may be attributed to the concomitant presence of thymol in the superfusate. [ABSTRACT FROM AUTHOR]

1 Concentration-dependent effects of thymol (1-1000 μM) was studied on action potential configuration and ionic currents in isolated canine ventricular cardiomyocytes using conventional microeleclrode and patch clamp techniques. 2 Low concentration of thymol (10 μM) removed the notch of the action potential, whereas high concentrations (100 μM or higher) caused an additional shortening of action potential duration accompanied by progressive depression of plateau and reduction of V[sub max]. 3 In the canine cells L-type Ca current (I[subCa]) was decreased by thymol in a concentration-dependent manner (EC[sub50]: 158±7 μM, Hill coeff.: 2.96±0.43). In addition, thymol (50-250 μM) accelerated the inactivation of I[subCu]. increased the time constant of recovery from inactivation. shifted the steady-state inactivation curve of I[subCu] leftwards, but voltage dependence of activation remained unaltered. Qualitatively similar results were obtained with thymol in ventricular myocytes isolated from healthy human hearts. 4 Thymol displayed concentration-dependent suppressive effects on potassium currents: the transient outward current. I[sub10] (EC[sub50]: 60.6± 11.4 μM, Hill coeff.: 1.03±0.11). the rapid component of the delayed rectifier. I[subKr] (EC[sub50]: 63.4 ± 6.1 μM. Hill coeff.: 1.29 ± 0.15), and the slow component of the delayed rectifier. I[subKs] (EC[sub50]: 202±11 μM, Hill coeff.: 0.72 ± 0.14), however. K channel kinetics were not much altered by thymol. These effects on Ca and K currents developed rapidly (within 0.5 min) and were readily reversible. 5 In conclusion, thymol suppressed cardiac ionic channels in a concentration-dependent manner. however, both drug-sensitivities as well as the mechanism of action seems to be different when blocking calcium and potassium channels. [ABSTRACT FROM AUTHOR]